In recent years, IEEE 802.11g and IEEE 802.11a are becoming remarkably popular as standards for high-speed wireless communication systems that utilize a 2.4 or 5 GHz band. Such wireless communication systems implement a transmission speed up to 54 Mbps by employing an OFDM (Orthogonal Frequency Division Multiplexing) method that stabilizes the characteristics of the systems in multipath fading environments.
In the future, the number of services utilizing a higher-speed wireless communication system will probably increase, and therefore, a great increase in the number of terminal devices in such a wireless communication system is expected.
However, since the frequency bands applicable to communication are finite, an increase in the number of terminal devices causes lack of frequency channels. As a result, no frequency channels may be usable by some terminal devices and the relevant users will not be able to perform communication utilizing the terminal devices.
Therefore, recently, multiuser MIMO transmission has been receiving maximum attention as a technique of increasing the transmission speed and implementing a larger-scale system. FIG. 10 is a diagram that shows the structure of a wireless communication system to which the multiuser MIMO transmission technique is applied.
In a multiuser MIMO transmission technique applied to a wireless communication system as shown in FIG. 10, different independent signals are sent at a common frequency and timing via N transmission antenna elements (N is a natural number greater than or equal to 2) of a base station apparatus 11 (as an access point (AP)) to U terminal devices (i.e., stations (STAs)) 12-1 to 12-U (U is a natural number greater than or equal to 2) as communication partners, each having M(u) antenna elements (u=1, . . . , U).
In the above process, the whole of the U reception antenna elements is regarded as a huge reception array so as to improve the downstream throughput.
The multiuser MIMO transmission technique may be a ZF (Zero forcing) method or a MMSE (Minimum mean square error) method (see Non-Patent Document 1).
In such communication techniques, a base station apparatus as a sending device acquires channel state information, which indicates propagation characteristics between antenna elements of the present apparatus and antenna elements of individual terminal devices, and computes a transmission weight based on the acquired channel state information
Generally, in order that the base station apparatus 11 acquires the channel state information before sending signals to the plurality of communication partners by means of the multiuser MIMO transmission technique, each terminal device 12 estimates the channel state information (i.e., transfer function) in advance and feeds the information back to the base station apparatus 11. Based on the relevant information, the base station apparatus 11 computes the weight to perform the multiuser MIMO transmission.
If TDD (Time Division Duplex) is performed between the base station apparatus and the terminal devices 12, the base station apparatus 11 utilizes a known signal sent from each terminal device 12 to estimate the channel state information for the upstream link. The estimated information is corrected utilizing a premeasured calibration value and the corrected channel state information is utilized by the base station apparatus 11 to compute the transmission weight so as to perform the multiuser MIMO transmission (see, for example, Non-Patent Document 2).